Design of Low Voltage, Low Power (IF) Amplifier Based-On MOSFET Darlington Configuration

This paper presents a different approach of Intermediate Frequency (IF) amplifier using 0.18 μm MIETEC technology channel length of MOSFET Darlington transistors. In contrast to Bipolar conventional Darlington pair, a MOSFET Darlington configuration is employed to reduce supply voltage (VDD) and DC consumption power (Pc). The frequency response parameters of the proposed design such as bandwidth, gain bandwidth product, input/output noises and noise figure (NF) are improved in proposed (IF) amplifier. Moreover, a dual-input and dual-output (DIDO) IF amplifier constructed from two symmetrical single input and single output (SISO) (IF) amplifier is proposed too. The idea is to achieve improved bandwidth, and flat response, because these parameters are very important in high frequency applications. Simulation results that obtained by P-SPICE program are 1.2 GHz Bandwidth (BW), 3.4 GHz (gain bandwidth product), 0.5 mW DC consumption power (Pc) and the low total output noise is 12 nV Hz with 1.2 V single supply voltage.


Introduction
The communication market has been growing very fast during the last decade especially for mobile communication systems.The low power, low voltage and low noise (IF) amplifier is one of the most essential building blocks in the communication circuits.It can be found in the almost of the commercial and military receivers [1].Several architectures of (IF) amplifier such as operational amplifier and Darlington pairs have been reported [2,3].The most common used Darlington pair consists of an emitter-follower and a common-emitter bipolar transistors [3,4].However, a major drawback is encountered with its performance.At higher frequencies its response becomes poorer than that of a single transistor amplifier [5].To overcome this problem, a number of modifications are attempted in Darlington pair amplifiers either by adding some extra biasing resistances in the circuit or by using Triple Darlington topology the earlier published Darlington amplifiers [5][6][7][8].All previous work still suffers from high DC consumption power due to high value of collector current, high noise and limitation in bandwidth.In this paper, a simple circuitry high performance single input and single output (SISO) (IF) amplifier based on (0.18 um) channel length MOSFETs Darlington configuration is proposed.The proposed amplifier used small channel-length (0.18 μm) to overcome the problems in consumption power, limitation in bandwidth, and inter-electrode capacitances.Because the small channel length of MOSFETs reduced the effect of inter-electrode capacitances at high frequency operation of MOSFETs.Besides that the reducing channel length will reduce the value of threshold voltage of MOSFETs and capable the designer to use small value of supply voltage and supply current.A dual-input and dual-output (DIDO) (IF) amplifier constructed from two symmetrical (SISO) (IF) amplifiers is proposed in this approach.The proposed dual-input and dual output (DIDO) (IF) amplifier is important in vast area of mobile applications such as multiband, wideband, and high-isolation multiple-input multiple-output techniques.

Design of Single Input and Single Output (SISO) IF Amplifier
Figure 1 shows the schematic circuit diagram of the proposed (IF) amplifier based on MOSFET Darlington configuration.The proposed amplifier constructed from two stages using NMOS transistors, biasing voltage and biasing resistors.The values of biasing resistors includ- ing R G1 , R S1 , R D2 , and R S2 can be calculated using dc analysis of two stages separately.In this design, the value of dc consumption power is very important for low power consideration, so that we have to choose suitable values of supply voltage V DD and biasing current I D .We can find the value of aspect ratios of two NMOS transistors using drain current equation for MOSFET as follows [9]: where, K n is a channel length modulation parameter, V GS is the gate to source voltage and V T is the threshold voltage of MOSFET transistors.
The change in drain current that will results from a change in gate-to source voltage can be determined using the transconductance factor   m g in the following expression: The transconductance factor is important in calculating the value of voltage gain, current gain and gain-bandwidth product (GBP) of the (IF) amplifier.
The ac small-signal equivalent circuit of the design configuration is shown in Figure 2. The overall voltage A V of the small signal equivalent circuit is given by: We note that the value of A V depend on the value of second term of Equation (3), if the value of factor , the value of overall voltage gain is given by: To investigate the effect of the external capacitors (blocking capacitors  1 and by pass capacitor (C P )) and internal capacitors C GS , C GD , and C DS on frequency response of the (IF) amplifier in low and high frequencies following expressions of cut off frequencies is used: In low frequencies the largest cut-off frequency of the amplifier is determined by bypass capacitor (C P ) and equivalent resistor as illustrated in Equation ( 5).The value of equivalent resistor is given by: where, R eq is the equivalent resistance looking by source terminal of MOSFET in (IF) amplifier .
The analysis of the high-frequency response of the proposed (IF) amplifier using high frequency equivalent circuit is shown in Figure 3.
1) The cut-off frequency of the overall voltage gain for the input circuit is defined by following expression: where Thi is the Thevenins resistance of input circuit and 1 in is the total input capacitance included inter electrode  

GS wi
2) The cut-off frequency of the overall voltage gain for the output circuit is defined by following expression: where R Tho is the Thevenins resistance of output circuit and C o is the total output capacitance included inter electrode capacitance (C GD ) and wiring capacitance (C wo ).
From Equations ( 7) and ( 8), we note that the interelectrode capacitances of MOSFET transistors are play important role in determining the frequency response of (IF) amplifier in high frequencies.These capacitances defined by gate dimensions of MOSFET, so that we can extend the value of upper-cut-off frequency by suitable choose of technology 0.18 μm channel-length of MOS-FET.Our design confirms this concept as we see in simulation results.The gate dimensions of NMOS transistors and biasing currents of the proposed (IF) amplifier are given Table 1.

Design of Dual-Input, Dual-Output (DIDO) (IF) Amplifier
The dual-input, dual-output (DIDO) (IF) amplifier plays important role in several mobile applications such as multiband, wideband, and high-isolation multiple-input multiple-output techniques [10].The proposed (DIDO) (IF) amplifier is based on simple circuitry approach with high performance parameters compared with convenional amplifiers.The design idea of proposed dual-input t

Simulation Results
The frequency response of the proposed amplifier that characterized by flat voltage gain and wide bandwidth is shown in Figure 5, where the maximum value of the voltage gain is 7.6 dB. Figure 12 shows the harmonic measurements of DIDO (IF) amplifier.In this figure, the value of harmonic distortion is increased due to increasing in frequency.To decrease the effect of harmonic distortion source degeneration technique can be used.
Table 2 shows the performance parameters of the proposed (IF) amplifier compared with other designs in previous works.The simulation results of the proposed amplifier verify the excellent improvement in dc consumption power, and low supply voltage.

Figure 2 .
Figure 2. Small-signal equivalent circuit of MOSFET Darlington configuration.

Figure 3 . 3 (
Figure 3. High frequency ac equivalent circuit for proposed (IF) amplifier.Table 1. Gate dimensions and biasing currents of MOSFETs for proposed (SISO) (IF) amplifier in Figure1.Gate dimensions and basing currents

Figure 6
shows the phase response of the voltage gain.The input and output noises of the proposed (IF) amplifier is shown in Figure7.As can be seen in measurement, values of input/output noises are increased with increasing the frequency of output voltage and current but still in acceptable range due to high noise immunity of MOSFET technology that used in proposed (IF) amplifier.The value of output current delivered to the resistive load used in proposed (IF) amplifier is decreased as R L varied from 10 kΩ to 100 kΩ.The value of output current is varied from 560 μA to 10 μA as load resistor is varied from 10 kΩ to 100 kΩ as can be seen in Figure8.Figures 9 and 10 show the frequency response (phases) and (magnitudes) of the dual-input and dual-output (DIDO) (IF) amplifier.We note that the frequency response in same values for both outputs (positive and negative) with phase difference is 180˚.Moreover, the transient time response shown in Figure 11 of DIDO (IF) amplifier for both outputs (positive and negative) are in same value with phase difference is 180˚ too.Figures 9-11 prove that the DIDO (IF) amplifier operates as we expected in theoretical background.

Figure 5 .
Figure 5. Frequency response (magnitude of voltage gain) of single (IF) amplifier.

Figure 6 .
Figure 6.Frequency response (phase of voltage gain) of single (IF) amplifier.

Figure 7 .
Figure 7. Input and output noises of proposed (IF) amplifier.

Figure 8 .Figure 9 .Figure 10 .Figure 11 .
Figure 8. Frequency response of output current as R L is varied from 10 kΩ to 100 kΩ of the pr osed (IF) amplifier.op

Table 2 . Summar
IF) amplifier based on Darlington configuration is designed in this work.Minimum channel length (0.18 μm) of MOSFET Darlington transistors is used to improve the performance of MODFETs in high frequency operation, and to reduce the effects of parasitic capacitance of MOSFETs in high frequency operation.Moreover, the decreasing of channel length of MOSFETs reduces the value of power supply of the circuit and the consumption power because the threshold voltage of MOSFETs has become smaller.A dual-input and dual output (DIDO) (IF) amplifier constructed from two symmetrical single input single output (IF) amplifier is also proposed in this paper.A wide bandwidth around (1.2 GHz), wide gain bandwidth product around (3.0 GHz), low supply voltage (1.2 V), and low consumption power around (0.5 mW) are achieved in this proposed design.The drawback of this design is low voltage gain around (7.6 dB), but it can be increased using feedback technique or using triple stage Darlington configuration.